The real Claremont chert

The Claremont chert actually has the official name Claremont Shale, and I should stop calling it Chert-with-a-capital-C. The rock unit was first named by Andrew Lawson, UC Berkeley’s memorable professor of geology who masterfully conducted and wrote up the scientific studies of the 1906 earthquake. (Lawson has the mineral lawsonite named for him, too. I hope to find some in Oakland some day.) Lawson named the Claremont Shale because the unit is predominantly shale, even though in Claremont Canyon, as in the rest of Oakland, it’s quite cherty.

When geologists name a rock unit for the first time, they designate a type locality and take the name from that. They also designate a type section, a specific place where anyone can visit and check the definitive example of the XYZ Formation. They draw a detailed stratigraphic column from the type section—it’s sort of like an engineering drawing, very formal. And they publish a detailed description of the unit and the stratigraphic column in a reputable journal or government-issued publication. The type section is supposed to display the top and the bottom of the formation and, ideally, everything between. The US Geological Survey has a panel of experts who do nothing but keep track of geologic names, both for rock formations and for the Eocene/Jurassic/etc. geologic time units. I used to work across the corridor from one of them, and he was quite the specialist.

So Claremont Canyon is the type locality for the Claremont Shale. Lawson’s original description is in US Geological Survey Geological Atlas Folio 193, published in 1914. The UC Geology Library probably has a copy. And this exposure of the chert along upper Claremont Avenue must make up part of Lawson’s stratigraphic column in that old atlas.

12 Responses to “The real Claremont chert”

I don’t know about finding lawsonite in Oak-town. Are there any eclogite grade rocks in the East Bay? Ring Mtn across the bay, of course, is the type locality, although finding lawsonite there is not not so obvious.

I tried to visit the type locality of lawsonite, at least as I understood it from what my informant said, but it was heavily overgrown. No, I don’t expect to see any eclogite in Oakland; fortunately lawsonite doesn’t require such extreme conditions.

There are several prominent outcrops of very hard, high-grade metamorphic rock in the upper El Cerrito area. I don’t remember if the rocks had garnet in them, but they might, and anyway, they are very nearly eclogite.

I’m trying to lead my son and a couple of other kids on a science project (6th grade) to characterize several outcroppings of this chert on Skyline and Elverton (in between Thornhill and Shepard Canyon). Not quite outcroppings, they are exposed due to road building.

I was wondering if there are any methods for characterizing the strata that would be accessible to us. The crux of their project will be to determine what is making these rocks look different in appearance from each other (weathering, age, composition and so on). I’m a chemist by trade, so basic wet chemical methods are a possibility.

That’s an interesting challenge. The simplest chemical test is the acid test for carbonates (vinegar works), but Oakland has almost none of those rocks. The other basic test suitable for children is hardness, in which case the shales along that stretch of road, if you can get a decent piece at all, are softer by far than the chert. Another useful thing to do might be crumbling the different sorts of rocks. Shales and sandstones would yield clay particles and sand grains, whereas the chert would need to be (carefully) hammered. Examine these particles with a magnifier and describe them, compare them with beach sand or washed soil sediments from the yard. Try chewing the shale–much of it is absolutely without grit.

If you have the materials to dissolve silica, you might demonstrate the presence of microfossils in the chert. I’ve never done it, but with vigilance it shouldn’t be difficult.

I have a reference that says the yellow rock might be dolomite, I suppose with a bit of iron in it as well? The best way I know of to dissolve silica is HF. I don’t know if that’ll get the carbonates of the fossils, assuming that’s what the fossils are made of.

Of course, dolomite is a carbonate, so that’s an easy one to test. The dark gray shale, I think, is in pastry thin layers in between most of the yellow stuff. The exposed areas we’ve found have various degrees of weathering and some crumble to dust with little encouragement.

Hi Andrew,
We took samples from thick and thin strata layers in a road cutout just to the left of the entrance to Huckleberry, on Skyline. The thin layered section had layers less than a centimeter thick while the thick ones were over 5 cm thick. We baked them in an oven (little weight loss, but it had been raining, playing it safe) and then broke them into small pieces of about a centimeter in size and soaked them all in hydrochloric acid for a week. None of them showed much weight loss but the thick layer of strata, upon breaking, was gray on the inside rather than the light orange to white on the outside. The thin layers were light orange to dark gray.

After a week in the acid all light color was removed from all samples. The thick layer was uniformly gray. The thin layer was composed of two rocks, tan and dark gray. The thick layer was the hardest of all, even after the acid. The dark gray was next, and the tan the softest, and was the only one that could be broken in your hands.

Based on these characteristics we believe that the three types of rocks are:

The thick stratum is chert- hard, gray, with a trace of limestone and iron for color.

The thin, flaky stratum was composed of alternating layers of dark hard siltstone (felt gritty when rubbed against teeth) and tan shale with a bit of limestone and iron for color.

We are going to characterize some rocks from other locations. One of the first we tried in acid was light orange and floated at first, fizzed, turned brown and sank. When we picked up the remaining brown rock and it promptly turned to mud in our hands, so we know there are some rocks in the area with a decent amount of limestone or dolomite in them.

One more weekend to go on the science project. You have been an excellent resource!

A hardness test using your pocket knife would also be helpful. If you scratch the surface of the rock with the tip of your pocket knife, it will not leave a clear mark on the fresh surface (i.e., not weathered) of a chert chert, but it will leave a clear mark on limestone.

I am a geologist at Berkeley and just completed a class studying the Berkeley hills and all the different strata. Just wanted to offer another perspective on the name. It is formally known as the Claremont Formation, but it is typically referred to as the Claremont Chert since chert is the main component of the formation. As you may well know in addition to the shale interbedding there are also some thin beds of limestone.

Forgot one thing. Your best best for Lawsonite is not in the east bay. It is almost always associated with the blueschist facies of metamorphic rock along with glaucophane. Although it does occur as a altered mineral in Gabbros and diorites, but that is much less common. Good luck finding it. Its cool looking stuff in thin section.

Jim, I took my cue from Russ Graymer’s geologic map of the Oakland area (MF-2342), where he refers to “the formally accepted name Claremont Shale” citing Lawson 1914. Adding to the confusion is that the Claremont is actually a local piece of the enormous Monterey Formation.

As for blueschist, we do have it in Oakland. But I’m hoping to find lawsonite in large enough grains to identify it without a thin section, not having my own pet mike.

Hi Guys,
Been a while, but I wanted you to know that my son and his partners got an A on the science project. It was a lot of work at the end to pull it all together (6th graders, go figure). They had to give a presentation about it that we didn’t realize until the morning of, but since I wanted them to do the work and understand what they were doing they were able to explain it well.

My son has had his fill, but I’m still interested in this stuff. Keep the posts coming. Nice to know what we have in the area and to be able to identify it.